The IP Multimedia Subsystem (IMS) is a sub-network created by the 3GPP wireless standards body. IMS is intended as a fully multi-vendor interoperable system which can support delivery of multimedia services to wireless users on a very large scale. Although initially developed for delivery of services to wireless networks, however, it is intended that IMS will also be capable of delivering the multimedia services in an access independent manner i.e. independent of the technology (cellular wireless, WLAN, wireline or other access technology) used by the user to access network services.
FIG. 1 shows a schematic diagram of the IMS network 100 as proposed in 3GPP R5. The diagram shows only a selection of the possible network elements: the Home Subscriber Server (HSS) 101, an Application Server (AS) 102, a Serving Call Session Control Function (S-CSCF) 103, an Interrogating Call Session Control Function (I-CSCF) 104 and a Service Location Function (SLF) 105. The elements are connected by an IP network 106.
An important component of the IMS is the Home Subscriber Server (HSS) 101 which is a database for the network which holds variables and identities for the support, establishment and maintenance of calls and sessions made by subscribers. This includes the subscriber's International Mobile Subscriber Identity (IMSI), security variables and location information. In addition to storing this information on users (also referred to as subscriber profiles) the HSS maintains current subscriber status information, provides service filter criterion, ensures the authenticity of the subscriber and facilitates access independence (e.g. WLAN, wireline).
The S-CSCF 103 provides the session control services for a user and performs functions such as the management of mobile registrations, maintaining of the session, interaction with other services, charging and security. The I-CSCF 104 allows subscribers of the network operator, or roaming subscribers to register.
The network may contain more than one HSS (as shown in FIG. 1) and these may be provided by different vendors. In the situation where there are more than one HSS in the network, the SLF 105 can be used by the CSCF and AS nodes to determine which HSS holds the subscriber profile for a particular user.
Given the important role of the HSS and the large amounts of information that it may need to store, it is vital to provide a robust and scalable HSS solution.